The present invention relates to an image forming apparatus provided with a pre-transfer charger to improve the efficiency in transferring a toner image onto a copy sheet.
Generally, an image forming apparatus comprises a toner image forming unit for forming a toner image, and a
toner image transferring device for transferring the toner image onto a copy sheet dispensed from a cassette. The toner image forming unit has a photosensitive drum. Specifically, the copy sheet is transported near a surface of the photosensitive drum and the toner image is transferred onto the copy sheet from the photosensitive drum. The copy sheet is then discharged from the apparatus after the toner image is fixed on the copy sheet by a fixing unit.
Recently, there have been marketed image forming apparatus which are provided with a charger before a toner image transferring device to improve the transferring efficiency of toner image from the surface of the photosensitive drum onto the copy sheet. Such a charger has been generally called a pre-transfer charger.
FIG. 14 is a schematic diagram showing an arrangement of a photosensitive drum and its peripheral devices in a conventional image forming apparatus. A surface of a photosensitive drum 10 is positively charged at a specified level by a main charger 11, and exposed to a light image A reflected from an original document to form an electrostatic latent image. Thereafter, an unnecessary area of the charged surface of the photosensitive drum 10 is removed by a blank lamp 12.
There is provided a developing device 13 containing negatively charged toner particles. Negatively charged toner particles are electrically attracted to the positive latent image on the photosensitive drum surface to develop a toner image.
In synchronism with the exposure and developing operations, a copy sheet is transported to the photosensitive drum 10 by a pair of registration rollers 24. The copy sheet is guided through a sheet transport passage 25 up to the surface of the photosensitive drum 10. The sheet transport passage 25 is constructed by an upper guide plate and a lower guide plate. Upon being transported to the surface of the photosensitive drum 10, the copy sheet is applied with positive electric charges by a transferring device 14 to transfer the toner image onto the copy sheet from the photosensitive drum 10. The copy sheet bearing the toner image is separated from the surface of the photosensitive drum 10 by a separating device 15 which is driven by an alternating current.
A pre-transfer charger 101 is arranged before the sheet transport passage 25 with respect to the rotating direction of the photosensitive drum 10. The pre-transfer charger 101 is driven by an alternating current whose negative component is slightly greater than the positive component. The pre-transfer charger 101 is provided to remove positive electric charges just below the toner image on the surface of the photosensitive drum 10, and to increase the electrical level of the negatively charged toner image. The pre-transfer charger 101 improves the transferring efficiency of toner image onto a copy sheet. Also, the pre-transfer charger 101 assists the transferring device 14 in transferring the toner image onto a copy sheet: which cannot be sufficiently charged by the transferring device due to absorption of moisture in the air.
As described above, in an image forming apparatus provided with a pre-transfer charger, it is necessary to provide both a sheet transport passage and a pre-transfer charger in a limited small space between a developing device and a transferring device.
There has been a strong demand for a compact-sized image forming apparatus. However, the above-mentioned conventional construction, in which the pre-transfer charger 101 is simply provided between the sheet transport passage 25 and the developing device 13 to improve the transferring efficiency, has increased the size of image forming apparatus. This contradicts the above-mentioned demand. Accordingly, a new construction has been greatly demanded for a long time which makes it possible to provide a pre-transfer charger in an image forming apparatus without increasing the apparatus size.
Also, it has been known that driving of a charger generates ozone which gives adverse effects to a photosensitive drum and environment around the apparatus. Accordingly, there has been demanded to remove ozone which the charger generates.
However, a pre-transfer charger is, as mentioned above, provided in limited small space where a developing device, transferring device, sheet transport passage, registration roller pair are densely arranged. Accordingly, there has been another strong demand for a new construction which makes it possible to efficiently remove ozone from a pre-transfer charger without increasing the apparatus size.
Further, a pre-transfer charger is required to be held in a precise positional relationship with respect to a photosensitive drum and sheet transport passage in addition to being provided in a densely arranged space as mentioned above. Accordingly, the construction has been demanded which enables the operator to mount a pre-transfer charger in a specified position easily and accurately when replacing a broken pre-transfer charger with a new charger.